1. Field of Invention
The present invention relates to energy systems that include fuel cells and other primary power devices.
2. General Background
Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy without combustion and without harmful emissions. The basic physical structure, or building block, of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in FIG. 1.
In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode); the electrochemical reactions take place at the electrodes to produce an electric current, water and heat. A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, the reactants are regenerated by recharging, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as fuel and oxidant are supplied to the electrodes.
Individual fuel cells are typically connected in a series arrangement or stack in order to increase the overall potential and power output. The voltage and current output and therefore the power output of a fuel cell system depend on the number of cells in the stack, total active surface area and efficiency.
A common type of fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The electrolyte in this fuel cell is an ion exchange membrane (fluorinated sulfonic acid polymer or other similar polymer) that is an excellent proton conductor. The only liquid in this fuel cell is water, and thus corrosion problems are minimal. Because of the limitation on the operating temperature imposed by the polymer, usually less than 100 C, a H2-rich fuel is used. If a fuel cell is compared to an equivalent efficiency heat engine, the fuel cell does not need to achieve the large temperature differential to achieve the same Carnot cycle efficiency as the heat engine. This is because of the added energy gained from Gibbs free energy as opposed to simply the thermal energy. The resulting freedom from large temperature differentials in the fuel cell provides a great benefit because it relaxes material temperature problems when trying to achieve comparable efficiency.
The actual cell potential is decreased from its ideal equilibrium potential because of irreversible losses, as shown in FIG. 2. Multiple phenomena contribute to irreversible losses in an actual fuel cell. The losses, which are called polarization, overpotential, or overvoltage, originate primarily from three sources: 1) activation polarization, 2) ohmic polarization, and 3) concentration polarization. These losses result in a cell voltage (V) that is less than its ideal potential, E (V=E−Losses). The activation polarization loss is dominant at low current density. At this point, electronic barriers must be overcome prior to current and ion flow. Activation losses increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the entire range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites.
The fuel cell will normally operate in the linear portion of the curve shown in FIG. 2. Consequently the cell potential varies as the load changes. In the example of FIG. 2 the linear portion encompasses a potential range of about 0.5 volts to about 0.9 volts and the open circuit potential is even higher. While variation of 0.4 volts may not appear to be a large fluctuation it can have significant impact on the operation of a stack of fuel cells. As an example, assume a stack of 20 cells performing as the curve of FIG. 2 shows. Accordingly, this stack will have an output potential of 10 to 18 volts and may have an open circuit potential of over 20 volts.
Much modern electronic equipment that operates at a nominal 12 volts DC will not function when presented with a voltage outside a narrow range, e.g., 11-14 volts. Some equipment, when sensing a high potential at the power input will not turn on for safety and self protection reasons. Similarly, if a low potential is sensed at the power input the equipment will not turn on or shut itself down if the potential drops below the minimum level during operation.
While a fuel cell stack can be sized for an expected load range, there can be times in which a load fluctuation can result in a higher current than expected such as start transients. In this situation, as the current increases the stack potential is reduced and may be reduced below what is required for the equipment load to operate. Another circumstance is an extremely high load condition or short circuit. In this case the cell voltage can be driven close to zero and may result in damage to the cell if such a condition persists.
In the past, efforts have been made to mitigate voltage fluctuations and other variant conditions by using energy storage devices (ESDs), such as batteries or ultracapacitors (also called supercapacitors) across the output of the fuel cell and in parallel with the load, thereby creating a hybrid fuel cell/ESD system. Such hybrid systems can provide greater stability than fuel cell only systems, but the prior art systems have not provided adequate protection to the fuel cell, the ESD, and the load from voltage and current variations in the system. Therefore, there is a need for a hybrid fuel cell/ESD energy system that minimizes the potential for damage to its components from adverse voltage and current conditions.